Month: August 2012

ISDATE is a wonderful function used to test if the underlying value is a valid date. I have often found developers wink-their-eyes in shock over ISDATE’s non-deterministic behaviour. I hope this small presentation will help developers use ISDATE with care.

What this function does?

ISDATE function returns 1 if the value/expression under test is a valid datetime and returns 0 for all other values.

Syntax

ISDATE (value/expression under test)

Let us know take a deep-dive into application of this function. Though it would be very easy for me to list down what is valid (when function returns 1) and what is invalid (when function returns 0) date; it would be better to apply and check how this function behaves in different scenarios (build over different application sets, given below, with varied complexity). These will build-up our learning base very well.

Application Set – 1

Below is the list of queries where ISDATE is applied to different expressions. For all the these cases, consider that dateformat is set to DMY and language is set to English (US).

As per ISDATE logic it is clear that for #1 and #2 the return value should be 1 and for all other it should be 0. But this is not the case!

#3 and #7 also have return value of 1. Why??

Because, SQL Server can implicitly convert expression (character string specifically) in #3 and #7 into valid datetime and for others (#4, #5 and #6) implicit conversion will fail with out-of-range conversion error. To demonstrate this, just try to execute the following code snippet in SSMS.

From above, picture about the behaviour of ISDATE is very clear now. It is also crystal clear that Datatime2 and Date data type are not valid arguments for ISDATE function. But care should be taken when datetime2 value is directly passed as character string in ISDATE argument. Here return value will be zero rather than an error message.

SELECT ISDATE('15-01-2012 00:00:00.0000000')
-- Returns 0

In nutshell, we can summarize our discussion with following points:

Behaviour of ISDATE function is dependent on default dateformat and language

It returns 1 when the string argument can implicitly be converted into valid datetime value. Increasing the seconds precision of the same string value to represent datetime2 format (or even if the seconds precision exceeds 3 significant digits) will return a 0 value.

Date and Datetime2 arguments are not allowed, they result in invalid argument error

Null values inside expressions also results in 0 as return value

Values such as text, ntext and image data types also return 0 value

I hope you enjoyed reading this post and must have admitted few facts to your memory.

The motivation to write this post came from SQLServerCentral Question of the Day. Here is the link to the question posted by me –> link

Ideally I want to assign value ‘A’ and ‘C’ to suppose partition-1, ‘B’ to partition-2 and any thing else to partition-3. Directly using partition range function over this value column will only allow value ‘B’ to go to partition-1 as it is between ‘A’ and ‘C’ which are part of partition-1. To do away with we need to create a computed-persisted column in this table. The computed column will give us an indicator, based on a logic we define, where the partitions would be assigned.

Below code snippet adds a computed-persisted column with logic to generate a new value which can be assigned to partition range function. Idea here is to generate a same indicator for all the values that we want to put in one partition.

ALTER TABLE list_partition
ADD partition_flag AS ( CASE value WHEN 'A' THEN 1 WHEN 'B' THEN 2 WHEN 'C' THEN 1 ELSE 3 END ) PERSISTED
SELECT *
FROM list_partition

Below code snippet does the following:
1) Create partition range function, it will create three partitions range for the following:
Partition 1 – Partition value less than and equal 1
Partition 2 – Partition value greater than 1 and less than equal to 2
Partition 3 – Partition value greater than 2
2) Create partition scheme and attach all the partitions to primary file group

CREATE PARTITION FUNCTION pf_list_partition ( INT )
AS RANGE LEFT
FOR VALUES ( 1 , 2 );
CREATE PARTITION SCHEME ps_list_partition
AS PARTITION pf_list_partition ALL TO ( [PRIMARY] )
--Partition scheme 'ps_list_partition' has been created successfully.
--'PRIMARY' is marked as the next used filegroup in partition scheme 'ps_list_partition'.

Having reached here, in order to partition the table we need to create a clustered index on the partition_flag using partition scheme – as shown below